The present invention relates generally to a method and apparatus for performing a work operation on limp sheet material and relates more particularly to apparatus having a table across which sheet material is held in a spread condition while a work tool in the apparatus performs a work opertion on the material.
Prior art apparatus for working on limp sheet material such as cloth, paper and the like, frequently includes a work table having a flat bed defining a support surface on which sheet material is spread. A work tool or instrument which is supported on a carriage above the support surface performs a work operation on the material. Typically, the carriage is movable over the support surface in one coordinate direction parallel to the support surface while the instrument, mounted on the carriage is movable relative to the carriage and the support surface in another coordinate direction parallel to the surface. Composite movements of the carriage and instrument allow the instrument to be translated to any coordinate position over the region of the support surface traversed by the carriage. Accurate positioning of the carriage and the instrument supported thereby may be achieved by numerical controls which operate either from an on-line data generator or from previously programmed data. The apparatus may be provided with a wide variety of work tools, such as for example, plotting pens or styluses, ink jet nozzles, light heads, tracking heads and cutting or drilling tools.
The work table of the prior art apparatus commonly includes a bed of material defining a support plane on which sheet material to be worked is laid in spread condition. The material comprising the bed may be any of a number of support materials, such as, for example, hard sheet steel having a smooth planar surface or, as described in U.S. Pat. No. 3,477,322, a resilient rubber platen. A characteristic that each of these support-surface or plane-defining beds has in common is that adjacent areas of the bed defining the material support plane are very close to, if not in contact with, one another so that every region of sheet material spread over the support surface is supported by a portion of bed material in engagement with one side of the spread sheet material. Although beds of this type may provide a firm support plane for the sheet material spread thereon, the bed material may represent a large portion of the total cost of the apparatus table. In manufacturing the tables, the support plane of the bed must be smoothed and precisely leveled within limited tolerances to cooperate with the tool. The bed must also be fabricated from materials that withstand shipping and handling, and care must be exercised to prevent damage to the bed surface when the apparatus is in and out of use.
The span of the material support surface defined by the beds of the prior art may be relatively short compared to the length of a strip of sheet material to be worked on. Therefore, to permit a work tool to complete a work operation on the entire strip of material, the material must be indexed or shifted across the support surface in segments by suitable indexing means. An example of an apparatus with indexing means for performing such an indexing operation is a plotting machine described in U.S. Pat. No. 3,844,461. Commonly, the bed remains stationary while the material is indexed relative to the support surface thereby creating friction between the support surface and the strip of material moved relative thereto. If the friction created during an indexing operation can be reduced, then the required power and the cost of components performing the operation will be reduced.
It is accordingly an object of this invention to provide an apparatus for working on limp sheet material and having an improved table which is less costly than tables of the prior art.
It is a further object of this invention to provide a method and apparatus having an improved table across which limp sheet material may be indexed in segments with much less friction than is created in the apparatus of the prior art.